Timing of sample collection for biological monitoring of occupational exposure.

The timing of sample collections for the biological monitoring of occupational exposure profoundly affects the resulting data. Sampling time with respect to the day in the working week and the end of exposure is crucial for measurements of rapidly excreted indicators of exposure. Owing to the cumulation of slowly excreted exposure indicators, timing of sample collection with respect to the duration of employment is essential. The steady state is established within a week, if the exposure indicator is excreted rapidly (with a half-life shorter than 45 h), or within months or years, if it is excreted slowly. In this study, exposure indicators are characterized by the elimination half-life. A monocompartmental model is used to calculate the biological levels at steady state and the duration of occupational exposure needed to reach the apparent steady state.